A. Field of the Invention
The present invention relates to a start control circuit and method for a brushless DC motor. More particularly, the present invention relates to a start control circuit and method for overcoming disadvantages such as dead point, head-sticking and insufficient initial starting torque, by producing a high starting torque at an initial time and accurately detecting a zero-crosspoint in sensorless brushless permanent magnetic DC motor drive applications.
B. Description of the Prior Art
Conventionally, in sensorless brushless permanent DC motor drive applications, such problems as deadpoint, where torque is zero, head sticking, in which the head of a disk sticks to the surface of the disk due to humidity and the like, low starting torque, and having no information about the position of the rotor, have been major difficulties in starting the motor normally.
Furthermore, start failures and start reliability are important issues in the hard disk drive (HDD) industry. To overcome such conventional difficulties, particularly, that of having no information about the position of the rotor, a technique of using electrical radians to detect the position of the rotor instead of Hall sensors is suggested in U.S. Pat. No. 5,254,914 by John C. Dunfield et al. and issued Oct. 19, 1993 and entitled POSITION DETECTOR FOR BRUSHLESS DC MOTOR.
In U.S. Pat. No. 5,254,914, one uses electrical radians to detect the rotor position instead of Hall sensors. However, there are two questions left unsolved. One is the accuracy of electrical radians in a motor drive IC, especially in a high speed precision DC motor. The other is that even if one can get the rotor position by electrical radians, that does not guarantee a high starting torque or a reliable motor start.
In addition, a technique of using a high-frequency commutation, in which a short current pulse is applied to each power phase of the motor and motor current conducted in response thereto is measured to monitor the position of the rotor, is suggested in U.S. Pat. No. 4,876,491 by John P. Squires et al issued Oct. 24, 1989 and entitled COMMUTATORLESS DC MOTOR FOR HARD DISC SYSTEM.
However, the high frequency commutation technique has a disadvantage in that it may cause reverse rotation of the motor when the high frequency commutation is applied during the motor starting period. Rather, it is preferable to detect a zero-crosspoint after starting the motor at a lower frequency than when the motor speed is in its normal state.
Still there remains a disadvantage in that the motor is slowly and abnormally rotated, since a high starting torque cannot be produced if the initial starting torque of the motor is insufficient, even when the motor is started after the zero-crosspoint is accurately detected.